DEFINITIONS OF TERMS USED. 19 



A region in which eruptive rocks abound is a region in which crystal- 

 line schists woukl naturally be expected to occur ; for here the conditions of 

 metamorphisni are best developed, — conditions that affect and metamor- 

 phose all the associated rocks, both eruptive and sedimentary, according to 

 their composition and physical structure. Eruptive rocks, whether in dikes 

 or lava flows, ashes or detritus of any kind, frequently possess the charac- 

 ters of crystalline schists ; must they therefore be regarded as being of 

 sedimentary origin ? The writer has seen dikes of crystalline schists cut- 

 ting directly across schistose conglomerates and other sedimentary rocks. 

 Was he to conclude that these dikes of schist were sedimentary, and had 

 been intruded in the form of schists; or ratlier that they were of eruptive 

 origin, — the original rock having been later metamorphosed into a schis- 

 tose rock ? He has also seen mica schists inclosed in distinctly eruptive 

 granites. If the law of association is worth anything, then should it not be 

 claimed that these schists were eruptive ? Ought not the evidence and the 

 lacts of the origin, and not the association, to be taken as proof of what that 

 origin was ? 



At this time, when the tendency is so strong to consider almost every- 

 thing the result of stratification, it seems necessary here to call attention to 

 some characters that for the most part are common to all rocks, whatever 

 may be their origin. While these characters are taken as proof positive of 

 sedimentation, in reality they have no bearing upon the question unless 

 they are exclusively confined to one class. 



Lamination in a rock is one of these characters ; which may be defined as 

 a structure, either original or superinduced in rocks of various kinds, causing 

 them to icnd to sj}lit into more or less parallel layers. 



This structure is very common in many eruptive rocks, especially those 

 of a fine-grained or glassy character, and which have become metamor- 

 phosed. In many rocks an aj^pearance of lamination is brought about by 

 the deposition of coloring matters in bands, and this pseudo-lamination even 

 has been oftentimes taken for stratification. 



Joint planes may be defined as fissures traversing rocks in a regidar or 

 irregular manner, independently of any other structural planes, induced 

 during the time of the consolidation of the rock or later, and dividing it 

 into masses of greater or less size.* 



These planes are frequently mistaken for bedding planes, both in 



* Faults aud fissure veius are but nioJificJ joiuts. 



